| Blue Lake | |
|---|---|
| The lake in early March with the original pumping station in the foreground. | |
| Location | Mount Gambier, South Australia |
| Lake type | Crater lake |
| Catchment area | Blue Lake Catchment Area |
| Basin countries | Australia |
| Max. length | 1,087 m (3,566 ft) |
| Max. width | 657 m (2,156 ft) |
| Surface area | 70 ha (170 acres) |
| Average depth | 72 m (236 ft) |
| Max. depth | 77 m |
| Shore length1 | None |
| Islands | None |
| Settlements | None |
| 1 Shore length is not a well-defined measure. | |
The Blue Lake in Mount Gambier, South Australia is a large monomictic lake located in an extinct volcanic maar associated with the Mount Gambier maar complex. It is one of four crater lakes on Mount Gambier. Of the four lakes, only two remain, as the other two (Leg of Mutton and Brown) have dried up over the past 30 to 40 years as the water table has dropped.
Conflicting dates have been estimated for its last eruption, of 28,000 years ago[1] and 4300 years ago.[2] If the more recent date is correct, this could be the most recent volcanic eruption on the Australian mainland.
Blue Lake is thought to be of an average depth 72 metres, but in places reaches 75 metres deep (250 feet). The crater rim measures 1,200 metres (3,937 feet) by 824 metres (2,703 feet), however, the lake itself measures 1,087 metres (3,566 feet) by 657 metres (2,155 feet). The bottom of the lake is 30 metres (100 feet) below the level of the main street of the nearby town. The Blue Lake supplies the town with drinking water, providing approximately 3,500 megalitres (2,800 acre·ft) per year from its 36,000 ML (29,000 acre·ft) store.
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Bathymetric surveys located the deepest point in the lake at 77 m in 1967.[3] Major diving exploration of the lake first occurred in 1985. Cave diver Peter Horne conducted temperature and visibility studies and made discoveries of a fresh water sponge species and other invertebrates.[3] This exploration also discovered "The Stromatolite Field", a collection of hollow rock formations that are found along the north-eastern perimeter down to a depth of 40 m.[3] In 2008, permission was granted by SA Water for another diving exploration of the central and deepest parts of the lake.[3] On this dive core samples from the calcite-silt covered lake bed were collected where water temperature drops to 14 degrees Celsius (57 °F).
During December to March, the lake turns to a vibrant cobalt blue colour, returning to a colder steel grey colour for April to November. The exact cause of this phenomenon is still a matter of conjecture, but it is generally considered likely that it revolves around the warming of the surface layers of the lake during the summer months to around 20 degrees Celsius (70 °F), causing calcium carbonate to precipitate out of solution and enabling micro-crystallites of calcium carbonate to form. This results in scatter of the blue wavelengths of sunlight. During winter the lake becomes well mixed, and recent research indicates that during this phase of the colour cycle the lake is somewhat murkier due to the redistribution of tannins and calcium carbonate particles throughout the lake. Solar elevation has also been found to influence the perceived colour of the lake. The movement of planktonic life-forms within the lake during the seasons and during the day may additionally play a part in the colour change.
Twelve kilometres (7 mi) south west of Mount Gambier, accessed by taking the turnoff by the Bellum pub, is the Little Blue Lake. It is one of a number of small nearby lakes whose waters also used to turn blue annually. However, pollution from agricultural fertilizers has increased the nutrient levels of these lakes to the extent that they now remain a year-round green colour. Swimming is discouraged and signs for the presence of blue-green algae have been erected.[4]
In July 1865 Adam Lindsay Gordon performed the daring riding feat known as Gordon’s Leap on the edge of the Blue Lake. A commemorative obelisk erected there has an inscription which reads: This obelisk was erected as a memorial to the famous Australian poet. From near this spot in July, 1865 Gordon made his famed leap on horseback over an old post and rail guard fence onto a narrow ledge overlooking the Blue Lake and jumped back again onto the roadway. The foundation stone of the Gordon Memorial Obelisk was laid on 8th July 1887.[5]